Liquid crystals, widely used at present as electro-optical media in display devices, are organic materials with anisotropic physical properties. The operation of the liquid crystal displays is based on the changes of the optical appearance of the liquid crystal in the display caused by an applied electric field.
One of the basic operational principle of liquid crystal displays and devices is the switching of the orientation of the liquid crystal molecules by an applied electric field that couples to the dielectric anisotropy of the liquid crystal (dielectric coupling). Such a coupling gives rise to an electro-optic response quadratic with the applied electric field, i.e. independent of the field polarity.
Another operational principle of practical importance is the one utilizing linear coupling between an applied electric field and a spontaneous polarization Ps present in ferroelectric liquid crystals.
There exist a number of different types of LCDs whose operation is based on dielectric coupling, especially dynamic scattering displays, displays using deformation of homeotropically aligned nematic liquid crystal, Schadt-Helfrich twisted nematic (TN) displays, super twisted nematic (STN) displays, and in-plane switching (IPS) nematic displays.
Another type of LCDs is the surface stabilized liquid crystal (SSFLC) display that operates on the basis of the linear coupling between the applied electric field and the spontaneous polarization Ps. The applied electric field could also give rise to a spontaneous polarization that linearly couples to the field. The deformed helix ferroelectric (DHF) liquid crystal displays and the antiferroelectric liquid (AFLC) displays are based on this principle.
For modern applications, an LCD should possess several important characteristics, such as a high contrast and brightness, a low power consumption, a low working voltage, short switching times, a low viewing angle dependence of the contrast, a grey scale or bistability, etc. The LCD should be cheap, easy to produce and to work with. Non of the prior-art LCDs is optimised concerning all the important characteristics.
In most of the conventional nematic liquid crystal displays, operating on the basis of the dielectric coupling, the electric field is applied normally to the liquid crystal bulk layer. These displays are usually slow, and nearly all suffer from non-satisfactory angular dependence of the contrast due to the switching of the liquid crystal molecules in a plane formed by the initial orientation of the molecules and the applied electric field (so-called out-of-plane switching). There is also another type of LCDs with in-plane switching, in which the electric field is oriented parallel to the liquid crystal bulk layer. These displays exhibit a very small angular dependence of the image contrast but the brightness and the switching times are not satisfactory.
Next, the other type of LCDs operating on the basis of the linear coupling between an applied electric field and the spontaneous polarisation in ferroelectric liquid crystals will be considered giving a short general description of the smectic liquid crystals with emphasis on ferroelectric liquid crystals and their application in the surface stabilised ferroelectric liquid crystal (SSFLC) display.
In a smectic liquid crystal, the molecules are arranged in adjacent smectic layers. The Smectic A and Smectic C phases are the two most important representatives of these “layered” phases of the liquid crystalline materials. In the Smectic A phase, the molecules are oriented along the smectic layer normal (θ=0°), whereas in the Smectic C phase the molecules are tilted with respect to the smectic layers at angle typically in the order of 20°. Furthermore a smectic liquid crystal could be achiral (e.g. A or C) or chiral (e.g. A* or C*), where the term chirality means lack of mirror symmetry. It should be notified that the term “chiral” does not refer to the occurrence of a helical molecular ordering that may or may not appear as a secondary effect.
The chirality, i.e. the broken mirror symmetry of the constituent molecules, is a prerequisite for the appearance of ferroelectricity, i.e. of spontaneous polarisation Ps, in tilted smectic liquid crystals like Smectic C*. Ps is directed along the smectic layers, i.e. perpendicular to the molecular long axis, and it is sterically coupled to the molecules.
Ferroelectric phases exhibit not only the smectic liquid crystal materials built up of chiral molecules but also achiral smectic host materials doped with chiral dopants. Most of the available commercial ferroelectric liquid crystal materials are such mixtures. In these mixed systems it is much easier to combine the broad temperature range and low viscosity of the selected achiral smectic C host material with the large polarisation induced by the selected chiral dopants. The proper combination of these material parameters is of vital importance for the ferroelectric liquid crystal mixture.
In Smectic C*, the molecules, being free to rotate around a cone with apex angle β=2θ, adopt a helical ordering due to chirality with a helix axis along the smectic layer normal. However, the Ps is spiralling, thus resulting in self-cancelling of the local polarisation. Therefore, in a bulk of Smectic C* there will be no macroscopic polarisation present (Ps=0). However, if an electric field is applied along the smectic layers of the helical Smectic C* bulk, the field will couple to the permanent dipoles aligning them parallel to the field. As a result, the electric field will unwind the helical order inducing macroscopic polarisation of the bulk of Smectic C* liquid crystal.
There is another way of suppressing the helical molecular order in order to obtain a spontaneous polarisation Ps in the bulk of Smectic C*. This is done by using the solid surface/liquid crystal interactions instead of an external electric field. This is the so-called surface stabilised ferroelectric liquid crystal (SSFLC) concept according to which the Smectic C* bulk is aligned in book-shelf geometry, i.e. with Smectic layers perpendicular to the confining surfaces.
In SSFLC displays, an external electric field applied along the smectic layers, i.e. perpendicular to the substrates, will switch the molecules of the ferroelectric liquid crystal between two positions on the smectic cone. These two positions correspond to the directions of Ps at different field polarity. An important feature is that the “flipp-flopp” mechanism (“the Goldstone mode) is much faster than the rather slow dielectric coupling mechanism that switches liquid crystalline materials having no permanent polarisation, such as conventional twisted nematic liquid crystal displays. Moreover, with a proper surface treatment of the solid display substrates, the switching of the liquid crystal molecules in SSFLC displays can be bistable. In the single component FLCs as well as in the FLC mixtures, the Ps present a homogeneous distribution in the bulk of these materials.
As an alternative to FLCs, the liquid crystal material could be in the so-called antiferroelectric (AFLC) liquid crystal phase, which means that in the absence of electric field, the molecules in adjacent smectic layers possess an opposite tilt. In an AFLC display, the dark state is obtained when placing the display between crossed polarizer set parallel and perpendicular, respectively, to the smectic layers. At this arrangement and when there is no applied electric field, the AFLC display is in the dark state. Under an applied electric field, both −E and +E give the same bright state. Thus, the AFLC display exhibits a three-state switching behaviour.
Drawbacks of the surface stabilized FLC and AFLC displays include difficulties in orienting the chiral smectic phases and maintaining their orientation. The FLC displays in addition have a sticking image problem due to building surface charged layers. The power consumption of these displays is also relatively high. Yet another drawback is that the thickness of the liquid crystal layer in these devices must be of order 1-1.5 μm in order to obtain the unwound state of these materials. This requirement for small thickness makes the production of FLC and AFLC displays complicated, delicate and very expensive.
In the displays discussed above, the desired alignment of the liquid crystal layer is achieved by appropriate treatment of the confining solid surfaces like coating with organic or inorganic layers as well as using a mechanical buffing. In the absence of external fields, the initial liquid crystal alignment is defined by solid surface/liquid crystal interactions.
The orientation of the liquid crystal molecules at the solid surface is transferred to liquid crystal molecules in the bulk via elastic forces. For instance, near the substrate surface the liquid crystal molecules in general could be oriented perpendicular (homeotropic) or parallel (planar) to the substrate surface that impose the same alignment of the molecules in the bulk of the liquid crystal. Since the liquid crystals are strongly birefringent, any change in their alignment will result in a certain change in their optical performance as seen between suitable polarizers.
In the prior of art, there are in principal three different techniques for changing the optical performance of liquid crystals by accomplishing a new molecular orientation in the liquid crystals that differs from the initial alignment.
1. Reorientation by Application of an External Field
The first, most widely used technique for reorientating the molecules is to apply an external electrical field over the entire bulk liquid crystal layer. Due to direct coupling between the electric field and some of the liquid crystal material parameters, such as dielectric anisotropy and spontaneous polarization, the field will directly reorient the liquid crystal molecules in a new direction if their initial alignment does not correspond to a minimum energy of interaction of the electric field with some of the liquid crystal material parameters. However, in some cases the liquid crystal molecules near the solid surfaces are difficult to reorient by an electric field, due to the above-mentioned surface liquid crystal interactions, whereas the “bulk molecules” more remote from the surfaces are fairly free and therefore easy to reorient by the field.
2. Reorientation by Photo-controlled Command Surfaces
The second known technique for reorienting the molecules of a liquid crystal layer is to design one or both of the confining alignment surfaces as a photo-controlled “command surface”. Such a photo-controlled command surface is capable, when subjected to, for instance, UV light, to change the direction of alignment imposed by the surface on the liquid crystal molecules in contact with the surface.
The concept of “photo commanded surface” has been described by K. Ichimura in a number of papers overviewed in Chemical Reviews, 100, p. 1847 (2000). More specifically, an azobenzene monolayer is deposited onto the inner substrate surface of a sandwich cell containing a nematic liquid crystal layer. The azobenzene molecules change their conformation from “trans” to “cis” under illumination with UV light. The azobenzene molecules are anchored laterally to the substrate surface by the aid of triethoxysilyl groups. The trans-isomer of azobenzene moieties imposes a homeotropic alignment of the nematic liquid crystal (liquid crystal molecules oriented perpendicular to the substrate surface), whereas the cis-isomer gives a planar orientation of the liquid crystal molecules (parallel to the substrate surface). Hence, the conformational changes of the molecules in the alignment layer caused by the UV illumination will result in a change of the alignment of the nematic liquid crystal molecules. The relaxation to the initial alignment is obtained by illuminating the sample with VIS-light or simply by heating it to the isotropic state.
A drawback of using photo commanded alignment surfaces in order to switch a liquid crystal alignment between two states is the low speed. In addition, so far the photo commanded alignment surfaces are shown to be effective only in nematic liquid crystal devices. Another drawback is that the life-time of a device having a photo commanded alignment surface is reduced by the degradation processes that take place due to UV-irradiation. Moreover, the use of light as external factor for switching the alignment of the liquid crystal is not convenient especially for liquid crystal displays.
Therefore, the performance of liquid crystal displays is chosen to be controlled by an external electric field instead of by light. The electric field directly couples with the bulk liquid crystal and changes alignment, thereby changing optical characteristics of the liquid crystal display, such as light transparency, light absorption at different wavelengths, light scattering, birefringence, optical activity, circular dichroism, etc.
3. Reorientation by Electrically Commanded Surfaces ECS
The third known principle for reorientating liquid crystal molecules involves the use of so-called Electrically Commanded Surfaces (ECS). This principle is described in the published International patent application No. WO 00/03288.
The ECS principle, which is faster, is used to primarily control a ferroelectric liquid crystalline polymer layer. As mentioned above, LCDs based on the linear coupling between the spontaneous polarisation in ferroelectric liquid crystals and an applied electric field perpendicular to the confining substrates has a number of advantages over LCDs based on a dielectric coupling. More specifically, ferroelectric LCDs are much faster, they allow an in-plane switching of the optic axis with image contrast less dependent on the viewing angle and, at proper anchoring conditions, ferroelectric LCDs makes it possible to achieve a bistable switching. However, as pointed out already, there are several problems related to the use of ferroelectric liquid crystals in displays and devices.
According to ECS principle, a separate thin ferroelectric liquid crystalline polymer layer is deposited on the inner surfaces of the glass substrates confining a liquid crystal bulk material in a conventional sandwich cell. The ferroelectric liquid crystalline polymer layer acts as a dynamic alignment layer imposing a planar alignment on the adjacent liquid crystal bulk material. More specifically, when applying an external electric field across the cell—and thereby across the dynamic alignment layer—the molecules in the separate ferroelectric liquid crystalline polymer layer will switch. This molecular switching in the separate polymeric layer will, in its turn, be transmitted into the bulk volume via elastic forces at the boundary between the separate alignment layer and the bulk layer, thus resulting in a relatively fast in-plane switching of the bulk volume molecules.
The ECS principle seems to have all advantages of the ferroelectric liquid crystals avoiding at the same time most of their serious problems. However, there are still several requirements that the ECS materials should meet that make the preparation of ECS somehow quite demanding:    The ECS layer should be very thin (100-200 nm).    The ECS layer should preferably be well oriented in bookshelf geometry, i.e. with smectic layers normal to the confining substrates.
In order to keep the ECS layer and it operation intact, the material of ECS layer must be insoluble in the liquid crystal bulk material.
In addition to what is stated above, the following may also noted in order to appreciate the claimed invention:
In the conventional liquid crystal devices and displays, the dielectric anisotropy, as a physical property of the liquid crystal material determined by the liquid crystal molecular structure, has a homogeneous distribution everywhere in the liquid crystal layer confined between the supporting substrates. Thus, the dielectric anisotropy has everywhere the same sign (positive or negative) and magnitude. Hence, due to the dielectric coupling with an external electrical field, the liquid crystal molecules will be subjected to a torque everywhere with the same magnitude, if the applied electric field presents a homogeneous distribution in the liquid crystal layer.
Flexoelectricity is a general physical property of liquid crystal materials and refers to a polarisation that occurs as a result of splay and/or bent elastic deformation. Flexoelectric properties of a liquid crystal are characterised by the splay and bent flexoelectric coefficients. The sign and the magnitude of these coefficients are determined by the molecular characteristics as shape and net dipole moment. Hence, similarly to the dielectric properties, the flexoelectric properties have a homogeneous character, i.e. the magnitude and the sign of the flexoelectric coefficients representing these properties are the same everywhere in the liquid crystal.
Recently, the flexoelectric polarisation that occurs as a result of bend or/and splay elastic deformation(s) of the liquid crystal, has found applications in bistable nematic devices as well as in cholesteric devices. Flexoelectric polarisation of liquid crystals is strongly dependent on the molecular structure of constituting molecules. There are liquid crystals exhibiting zero flexoelectric polarisation when subjected to an elastic deformation of splay and/or bent kind.
A type of LCDs, in which the linear coupling between the applied electric field and the flexoelectric polarisation is implemented in addition to the dielectric coupling, is the bistable nematic liquid crystal display. In this type of display, the anchoring conditions for the liquid crystal to the two confining surfaces are strong and weak, respectively. The operation of this kind display is based on switching of the orientation of the molecules by the dielectric coupling at one field polarity that stays after switching-off the field. Application of electric field of the opposite polarity switches the molecules to their first position due to the linear coupling of the flexoelectric polarisation with the field. This position is hold by the liquid crystal after switching off the electric field. In this kind of liquid crystal display the flexoelectric properties of the liquid crystal are playing very important role.
Scalar order is a material parameter characterising the degree of order of the liquid crystal molecules along a preferred direction in the space called director. In the isotropic phase of the liquid crystal material, the scalar order parameter is zero. It increases its value when going from isotropic to nematic and to smectic phases. All physical characteristics of liquid crystal material as well as the liquid crystal/surface interactions strongly depend on scalar order parameter.
An applied electric field, with homogeneous distribution in the liquid crystal bulk layer, exerts a torque on the liquid crystal molecules being the same everywhere in this layer. In general, the liquid crystal molecules at the solid surface is difficult to reorient by an electric field due to liquid crystal/surface interactions whereas the bulk liquid crystal molecules, more remote from the surface are fairly free and therefore easy to reorient by the field. The strength of the applied electric field that requires to reorient completely the liquid crystal molecules at the surface is usually much higher than the one needed for the bulk liquid crystal molecules. Its magnitude depends on the anchoring strength and thus on the scalar order parameter.
Thus, in the prior art, the dielectric and flexoelectric liquid crystal properties have the same sign and magnitude everywhere in the liquid crystal layer (in the volume as well as at the liquid crystal surface interface). Therefore, an applied electric field, with homogeneous distribution in the liquid crystal layer, will impose on all liquid crystal molecules the same torque.
In the light of the above-mentioned desired properties of a liquid crystal device, and the above-mentioned different drawback of the known liquid crystal displays, a general object of the present invention is to accomplish an improved liquid crystal device, an improved method for manufacturing a liquid crystal device, and an improved method of controlling a liquid crystal device. The invention is not directed to displays only, but is useful in many other liquid crystal applications.